Currently, when my washer machine drains into the standpipe, about 3/4's of the way through draining, the water begins to back up out of the standpipe... the hose does fit somewhat snug into the standpipe, with no appreciable space for air. Is this because there is no air gap?? OR, is it because it doesn't fit tight enough?? How can I get this to stop backing up?

Sounds to me you have a partial blockage of the line... likely from lint and debriss coming out of the washer. I'm guessing you have no filter or screen to trap it coming out.


A washer pumps out that water a LOT faster than it comes out of the faucet... you are overwhelming the ability to drain past the partial blockage resulting in a backup.

I'd have that line snaked. And use something to trap that lint and stuff to reduce the chance of it happening again.

If this is something that has just started recently and you are using same washer, then smoothy is right on.

If this is anew washer on an old system of pipes, it is possible the power and volume of water to too much. Most washers today need 2 1/2'' pipe for the extra pressure.

You may have to make it a closed system. Parts you will need are available at any big box store.

Chuck

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the hose does fit somewhat snug into the standpipe,

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this tells me that you have a older home with a inch and a half standpipe. Today's more powerful washer pumps discharge more volume than the standpipe can handle. Now that we have that out of the way let's address your problem.
You do have a partial blockage further out down the line they should be snaked. But my personal solution for washer back up in a undersized standpipe is to make it a close connection either with duct tape or a trip question fitting. I have a older inch and a half standpipe and this is what I did to stop my backups. Take paying the hose connection to the standpipe forces the water through without a backup.
I have never had a problem myself and have never got a complaint back from anyone that has tried this more questions? I'm as close as a click. Good luck, Tom